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Dynaco ST-70 Wellbourne Modification

P

Pycroft

Member
Hi there...

I am a true tube noob...I've read the 65 page primer that's floating around, but know little about them. I have an opportunity to buy a Dynaco ST-70 with the Wellbourne Modification. I know a bit about the dynaco - 35 wpc tube power, vintage unit, I know what tubes it uses, etc. I'm not sure what 'Wellbourne Modification' means, or does to the original ST-70. Can someone comment on what specifically the modification is, and also if you have heard both the stock ST-70 and the modified, can you comment aurally the difference between the two? If you have one, I'd love to hear a critique of the amp.

Thanks much!

James
 
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Thanks for the link Phil. That's probably why my searches were unsuccessful for Wellbourne. I looked at the link, and was confused...I don't know if I actually saw the word Welbourne (Unless I missed it in my scan). Is it safe to say that the mod replaces some of the internal electronics with upgraded, better quality parts?

Would still love to hear about the aural differences!

Thanks much!

James
 
I've upgraded a few ST-70's with the Welbourne Labs mod, and the transformation is remarkable. It's the best mod I've ever heard for this series, and I highly recommend it.

The Welbourne Lab upgrade takes a rather pedestrian amp and turns it into a sparkler with lots of detail and sound stage, rivaling pricier products.

The Welbourne mod uses two tubes in a mu-follower configuration to replace each 7199. The gain is provided by 5751 tubes wired in parallel; a 6GH8 provides the active load for the gain stage.

Also, the mod adds quite a bit of filter capacitance to the circuit.

Buzz
 
it replaces almost all electronicparts... :)
i'd call it a new amp using chassis and trannies of a ST-70...
 
The Welborne card is but one of many input variations you can get to update the front end of the ST-70. I have installed and heard several but not that one. This includes replacing the caps and resistors of the original circuit with new, modern pieces. I think it all comes down to two things with the ST70. The power supply is it's weak link so whatever you can do to beef it up helps. With the front end it's a matter of which flavor you like. I modified the original circuit to run 6p3s-es outputs, a Russian 6l6 equivalent.
 
I used to use the Welborne mod in one of my ST-70s. In my opinion it was not worth the expense and complexity of installation.

It appears that the complexity issue has been addressed (at least somewhat) by the newest revision that has pretty much everything on one circuit board. The older versions were much more difficult to install.

Sonically, I didn't think it was much of an upgrade (if at all) over a stock ST-70 (rebuilt with all fresh parts) or the DiyTube driver board that I use now. There are several other boards out there that -- based on what I've read -- would also be very good candidates and better values for the $ (e.g. the VTA driver board about which I've never read anything bad).

Not that the Welborne was bad, mind you...just not worth the $ and effort in my opinion. YMMV.

Hope this helps.

Joe
 
Oh, it's a complete bear to install (like building a ship in a bottle) but IMHO, there is NO comparison between the stock ST-70 and the Welbourne mod. I've built a couple of them.

A tech buddy of mine who has restored 3 Citation II's did a Welbourne and he agrees with me that it sounds better than the HK amp. I'm sure I'd get a LOT of disagreement with that statement!

So if you can get one at a good price and not have to install it yourself, you won't be disappointed.
 
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